1. Field of the Invention
This invention relates to a process for making nuclear magnetic resonance (NMR) apparatus and to the apparatus made thereby and more particularly to a novel process for making a radio-frequency coil for NMR imaging and spectroscopy.
2. Description of the Prior Art
It is well-known in the art that NMR images and spectra may be obtained from certain nuclei to determine the internal distribution and chemical form of the nuclei in a specimen. For many important applications, however, the chemical species being imaged is present in low concentrations. It is therefore desirable to increase the sensitivity of the imaging apparatus while maintaining a homogeneous magnetic field throughout the volume sample. Moreover, in microscopy and other very high resolution applications, the resolution is limited by factors including the small number of spins in a volume element and probe efficiency. Imaging efficiency, both in terms of sensitivity and resolution, can be increased by optimizing the signal-to-noise ratio (SNR) of the NMR apparatus. It has been stated that coil thermal noise is the dominant noise source in NMR microscopy [Cho et al., Nuclear magnetic resonance microscopy with 4-.mu.m resolution: Theoretical study and experimental results, Medical Physics, Vol. 15, 1988, pp 815-824, at page 818].
NMR imaging coils, which typically are filled with a sample to be imaged, must also be tuned to a resonant frequency that depends upon the applied magnetic field. It is thus necessary to provide a capacitance with which to resonate the coil's inductance.
Prior art coil designs exist to optimize SNR for specific applications. For example, U.S. Pat. No. 4,885,539 to Roemer et al. describes a volume RF coil assembly of cylindrical form, including one embodiment of "birdcage" form. Conductor gaps are bridged with serially-connected capacitive elements. Other prior art designs include solenoid, slotted tube resonator (STR), and loop-gap resonator (LGR) designs [Murphy et al., A Comparison of Three Radiofrequency Coils for NMR Studies of Conductive Samples, Magnetic Resonance in Medicine 12:382-389 (1989)]. These are large-volume RF coil designs for use with conductive samples. All of these designs require the use of discrete capacitors, either on or as part of the coil assembly itself (as in the Roemer et al. design and the LGR design) and/or some combination of chip capacitors and variable air capacitors.
Inductive coupling and tuning of resonant circuits are well known in the art of radio engineering and have more recently been described in connection with NMR probe circuits [Kuhns, Inductive Coupling and Tuning in NMR Probes; Applications, Journal of Magnetic Resonance 78:69-76 (1988)]. The arrangement disclosed in this reference avoids the use of conventional, high-voltage tuning capacitors, and is suitable for iron-core magnets. Instead, the sample coil is tuned by a fixed capacitor, which is chosen to resonate the sample coil slightly below the operating frequency. Other probe circuits shown in this reference use conventional capacitive tuning, include variable capacitors in conjunction with the sample coil, or require conventional porcelain chip capacitors to resonate the sample coil. However, a high-Q coil requires a low L/C ratio, making it difficult to tune such circuits by conventional variable capacitors, because the inductance of the connecting wires typically exceeds that of the coil. The solution suggested in Kuhns is a self-contained resonator formed from overlapping conductors, the capacitor being formed in the overlap by separating the conductors by a Teflon.RTM. tape.
Inductively-coupled NMR coils are also known [Decorps et al., An Inductively Coupled, Series-Tuned NMR Probe, Journal of Magnetic Resonance 65:100-109 (1985)]. In the coil described in this reference, a series capacitor is inserted in the middle of the NMR coil to further reduce the electric losses and the shifts of the resonance frequency due to the introduction of a living sample or to movement of the sample.
It would be advantageous in an NMR coil to be able to position matching and tuning capacitors directly beside the inductors to improve the signal-to-noise ratio and to be able to fabricate the NMR resonating circuit as close to the sample as possible to increase filling factor (i.e., the ratio of actual sample volume to the cavity volume). Moreover, since we have found that shape defects in the coils cause a significant reduction in signal-to-noise ratio, it would also be advantageous to have a method of manufacturing NMR coils without shape defects easily, inexpensively, and in large quantities. Such coils should be useful for a broad range of applications from spectroscopic NMR to NMR microimaging.